Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes. Naturally occurring technetium occurs as a spontaneous fission product in uranium ore or by neutron capture in molybdenum ores. The chemical properties of this silvery grey, crystalline transition metal are intermediate between rhenium and manganese. Many of technetium's properties were predicted by Dmitri Mendeleev before the element was discovered. Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese (Em). In 1937 technetium (specifically the technetium-97 isotope) became the first predominantly artificial element to be produced, hence its name (from the Greek τεχνητός, meaning "artificial"). Its short-lived gamma ray-emitting nuclear isomer—technetium-99m—is used in nuclear medicine for a wide variety of diagnostic tests. Technetium-99 is used as a gamma ray-free source of beta particles. Long-lived technetium isotopes produced commercially are by-products of fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because no isotope of technetium has a half-life longer than 4.2 million years (technetium-98), its detection in red giants in 1952, which are billions of years old, helped bolster the theory that stars can produce heavier elements. Characteristics Technetium is a silvery-grey radioactive metal with an appearance similar to that of platinum. The crystal structure of the pure metal is hexagonal close-packed. Atomic technetium has characteristic emission lines at these wavelengths of light: 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm. The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields but will assume random orientations once the field is removed. Pure, metallic, single-crystal technetium becomes a type-II superconductor at temperatures below 7.46 K. Below this temperature, technetium has a very high magnetic penetration depth, the largest among the elements apart from niobium. Technetium is placed in the seventh group of the periodic table, between rhenium and manganese. As predicted by periodic law, its chemical properties are therefore intermediate between those two elements. Of the two, technetium more closely resembles rhenium, particularly in its chemical inertness and tendency to form covalent bonds. Unlike manganese, technetium does not readily form cations (ions with a net positive charge). Common oxidation states of technetium include +4, +5, and +7. Technetium dissolves in aqua regia, nitric acid, and concentrated sulphuric acid, but it is not soluble in hydrochloric acid of any concentration. Reaction of technetium with hydrogen produces the negatively charged hydride TcH92− ion, which has the same type of crystal structure as (isostructural with) ReH92−. It consists of a trigonal prism with a technetium atom in the center and six hydrogen atoms at the corners. Three more hydrogen’s make a triangle lying parallel to the base and crossing the prism in its center. Although those hydrogen atoms are not equivalent geometrically, their electronic structure is almost the same. This complex has a coordination number of 9 (meaning that the Tc atom has nine neighbours), which is the highest for a technetium complex. Two hydrogen atoms in the complex can be replaced by sodium (Na+) or potassium (K+) ions. Value The base value of each unit of ranges between 15 and 50Ð per unit, with up to 2 units being found at any one time. Presence on Mars: Very Rare Category:Chemical elements